JPH0784271A - Antiferroelectric liquid crystal display device - Google Patents

Abstract

(57) [Summary] [Object] To provide an antiferroelectric liquid crystal display device capable of obtaining a high-contrast display having a large display brightness ratio between the ON state and the OFF state. [Structure] A pixel electrode 3 of an upper substrate 1 and a counter electrode of a lower substrate 2 have a highest convex portion and a lowest concave portion on a scanning locus at a scanning distance of at least 500 nm when the surfaces thereof are scanned in an arbitrary direction. An electrode having a surface roughness with a height difference of 20 nm or less is horizontally aligned by the alignment films 6 and 7 of both substrates 1 and 2, and the alignment film (alignment film on the lower substrate 2 side of the alignment films 6 and 7 is arranged). The liquid crystal molecules whose alignment state was regulated by the regulation film 7 were aligned in a good alignment state with almost no disturbance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antiferroelectric liquid crystal display device.

[0002]

2. Description of the Related Art Recently, as a liquid crystal display element, a ferroelectric liquid crystal display element and an antiferroelectric liquid crystal which are superior in high-speed response and wide viewing angle as compared with a generally used TN type liquid crystal display element. Research on an antiferroelectric liquid crystal display device using is performed.

In the antiferroelectric liquid crystal display device, a pair of transparent substrates, each having a transparent electrode formed on one surface and a horizontal alignment film formed thereon, are arranged with their electrode formation surfaces facing each other. It is configured by enclosing an antiferroelectric liquid crystal in.

In this antiferroelectric liquid crystal display element,
One in which the horizontal alignment films of both substrates are each an alignment control film that controls the alignment state of liquid crystal molecules (for example, a polyimide film whose film surface is rubbed in one direction), and only the horizontal alignment film of one substrate is aligned Some are regulated.

This antiferroelectric liquid crystal display element utilizes the stability of the molecular alignment state possessed by the antiferroelectric liquid crystal, and the antiferroelectric liquid crystal enclosed between the substrates is the above-mentioned alignment control film. It has a smectic layer structure oriented according to the alignment control direction (rubbing direction in the case of a rubbing-treated polyimide film), and has three stability of the molecular alignment state, and the alignment direction of liquid crystal molecules depends on the electric field. Change.

The first stable state is a state when an electric field having a strong unidirectional polarity is applied to the liquid crystal layer. At this time, the spontaneous polarization of the liquid crystal molecules acts on the applied electric field, and Liquid crystal molecules are uniformly arranged with a tilt angle in one direction with respect to the normal of the smectic layer structure.

The second stable state is a state in which an electric field having a strong polarity in the opposite direction is applied to the liquid crystal layer. At this time, the spontaneous polarization of the liquid crystal molecules acts on the liquid crystal layer in the opposite direction to cause the liquid crystal to move. The molecules are inverted, and all the liquid crystal molecules are uniformly arranged with the tilt angle tilted in the direction opposite to the first stable state with respect to the normal line of the smectic layer structure.

On the other hand, the third stable state is a state when no electric field is applied or when a weak electric field is applied. In this state, liquid crystal molecules are alternately reversed at the same tilt angle with respect to the normal line of the smectic layer structure. Arrange in a state of being inclined in the direction (arranged in alternate directions for each layer). Therefore, the average alignment direction of the liquid crystal molecules in the entire liquid crystal layer in the state where no electric field or a weak electric field is applied is the normal direction of the smectic layer structure.

The antiferroelectric liquid crystal display element is used by arranging polarizing plates on both sides thereof, and these polarizing plates have their transmission axes in the three stable states of the antiferroelectric liquid crystal. Are provided so as to be substantially orthogonal or substantially parallel to the normal line of the smectic layer structure, which is the reference line, and the transmission axes of both polarizing plates are substantially orthogonal to each other.

The above-mentioned antiferroelectric liquid crystal display element displays the antiferroelectric liquid crystal molecules by controlling the molecular alignment state into the above-mentioned three stable states, and does not apply a voltage between the electrodes of both substrates. In the state or in the state where the applied voltage is low, the liquid crystal molecules are arranged in the third stable state and the display is in the OFF (dark) state, and the unidirectional polarity or the reverse polarity O is present between the electrodes.
When the N voltage is applied, the liquid crystal molecules are arranged in the first stable state or the second stable state and the display is turned on (bright).

[0011]

However, in the conventional antiferroelectric liquid crystal display element, there is a large amount of light leakage in the OFF state in which the liquid crystal molecules are arranged in the third stable state, and therefore, there are the ON state and the OFF state. The problem is that the contrast, which is the ratio of display brightness, is low. An object of the present invention is to provide an antiferroelectric liquid crystal display device that can obtain a high contrast display.

[0012]

In the antiferroelectric liquid crystal display device of the present invention, the horizontal alignment film of at least one substrate is an alignment control film for controlling the alignment state of liquid crystal molecules, and the alignment control film is also provided. The lower transparent electrode has a surface roughness such that the height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm when the surface is scanned in an arbitrary direction is 20 nm or less. It is characterized by being.

[0013]

As described above, the transparent electrode under the alignment control film that controls the alignment state of the liquid crystal molecules has a height difference of 20 nm or less between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm. In the case of an electrode having a surface roughness of, the liquid crystal molecules on which the alignment state is regulated by the alignment regulating film are aligned in a good alignment state with almost no disturbance, and the liquid crystal molecules are in the third stable state. OFF arranged
Light leakage in the state is reduced. Therefore, according to the antiferroelectric liquid crystal display element of the present invention, a high-contrast display having a large display brightness ratio between the ON state and the OFF state can be obtained.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of an antiferroelectric liquid crystal display device. The antiferroelectric liquid crystal display device of this embodiment is of an active matrix type, and in one surface (lower surface) of an upper transparent substrate (hereinafter referred to as an upper substrate) 1 in FIG. Active elements (for example, thin film transistors) 4 are arranged in rows and columns, and one surface (upper surface) of a lower transparent substrate (hereinafter referred to as lower substrate) 2 is transparent to face all of the pixel electrodes 3. Counter electrode 5
Are formed. Although not shown, an address line for supplying a selection signal to the active elements 4 in each row and a data line for supplying an image data signal to the active elements 4 in each column are wired on the upper substrate 1.

Further, horizontal alignment films 6 and 7 are provided on the electrode formation surfaces of the upper substrate 1 and the lower substrate 2, respectively.
Of these horizontal alignment films 6 and 7, one of the substrates, for example, the alignment film 7 on the lower substrate 2 on which the counter electrode 5 is formed is an alignment control film that controls the alignment state of liquid crystal molecules, and the other substrate (upper). The alignment film 6 on the substrate 1 is a non-alignment control film having no alignment control function for liquid crystal molecules.

The arrangement regulating film 7 is made of, for example, a polyimide film whose surface is rubbed in one direction, and the non-arrangement regulating film 6 is made of a polyimide film which is not rubbed.

In the antiferroelectric liquid crystal display element, the upper substrate 1 and the lower substrate 2 are arranged with their electrode forming surfaces facing each other, and the both substrates 1 and 2 are provided with a frame-shaped sealing material 8. The antiferroelectric liquid crystal 9 is sealed in a region surrounded by the sealing material 8 between the substrates 1 and 2 by being bonded to each other via the antiferroelectric liquid crystal. The liquid crystal 9 has a smectic layer structure oriented in accordance with the alignment control direction (rubbing direction) of the alignment control film 7.

In the antiferroelectric liquid crystal display element, after the antiferroelectric liquid crystal 9 is enclosed between the substrates 1 and 2, it is heated to a temperature at which the enclosed liquid crystal is in an isotropic phase, and then the liquid crystal is formed. Is produced by performing a re-orientation treatment in which the liquid crystal 9 is gradually cooled to a temperature in the range of an antiferroelectric phase, and the re-orientation treatment causes the alignment state of the liquid crystal 9 to be aligned in an orderly layered structure. Become.

Polarizing plates 10 and 11 are arranged on both sides of the antiferroelectric liquid crystal display element, and the transmission axes of the polarizing plates 10 and 11 are the antiferroelectric liquid crystal 9 described above. The polarizing plates 10 and 11 are provided so as to be substantially orthogonal or substantially parallel to the normal line of the smectic layer structure, and the transmission axes of the polarizing plates 10 and 11 are substantially orthogonal to each other.

Next, the transparent electrodes (pixel electrode and counter electrode) formed on both the substrates 1 and 2 of the antiferroelectric liquid crystal display device.
3 and 5, the transparent electrodes 3 and 5 are formed by forming a transparent conductive film made of ITO or the like on the substrates 1 and 2 by a sputtering apparatus and patterning the transparent conductive film by photolithography. The surface roughness of these electrodes 3 and 5 is such that the height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm when the electrode surface is scanned in an arbitrary direction. The roughness is 20 nm or less.

FIG. 2 shows the surface roughness of the counter electrode 5 formed on the lower substrate 2 on which the array regulating film 7 is formed, observed on an arbitrary straight line by an atomic force microscope (AFM). The surface of the counter electrode 5 has irregularities as shown in the figure, but the height difference h between the highest convex portion A and the lowest concave portion B on the scanning locus within the scanning distance L of at least 500 nm is 20 nm or less. Therefore, the counter electrode 5 is an electrode having small surface roughness and high smoothness. The same applies to the pixel electrode 3 formed on the upper substrate 1.

The transparent electrodes 3 having such a surface roughness
5 can be obtained by selecting the sputtering conditions when forming the transparent conductive film by the above-described sputtering apparatus. And, in the antiferroelectric liquid crystal display element,
At least 50 transparent electrodes 3 and 5 on both substrates 1 and 2
Since the height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of 0 nm is 20 nm or less, the alignment film 6, 7 formed on the electrode has a surface roughness of 20 nm or less.
Has a high smoothness, and is therefore horizontally aligned by the alignment films 6 and 7 of both substrates 1 and 2, and the alignment film on the lower substrate 2 side (alignment regulating film) of these alignment films 6 and 7. 7
The liquid crystal molecules whose alignment state is regulated by are aligned in a good alignment state with almost no disturbance, and the liquid crystal molecules are aligned in the third stable state (state when no electric field or weak electric field is applied) OFF Light leakage in the state (dark display state) is reduced.

FIG. 3 shows the OF of the above-mentioned antiferroelectric liquid crystal display device.
A part of the dark display in the F state is shown enlarged.
The white portion in the figure is the portion where light leakage occurs. The light leakage shown in FIG. 3 is obtained when the thicknesses of the alignment films 6 and 7 of the substrates 1 and 2 are 50 nm, respectively.

Looking at the leakage of light in the OFF state in the conventional anti-ferroelectric liquid crystal display element, FIG. 4 shows an arbitrary straight line of transparent electrodes formed on both substrates of the conventional anti-ferroelectric liquid crystal display element. The surface roughness obtained by observing the surface state above with an atomic force microscope is shown.

As can be seen by comparing FIG. 4 with FIG. 2, the transparent electrode in the conventional antiferroelectric liquid crystal display element has a large surface roughness, and therefore the alignment film formed thereon has a large surface roughness. Since the smoothness of the film surface is low, the alignment state of the liquid crystal molecules is disturbed, and this disturbance of the alignment state causes light leakage in the OFF state.

FIG. 5 shows an active matrix type similar to the liquid crystal display element of the above embodiment, and the alignment film on the substrate on which the counter electrode is formed is an alignment control film for controlling the alignment state of liquid crystal molecules. In the conventional anti-ferroelectric liquid crystal display device, the alignment film on the other substrate is a non-alignment control film that does not have the alignment control function of liquid crystal molecules, and the thickness of the alignment films on both substrates is 50 nm. A part of dark display in the OFF state is shown in an enlarged manner, and a white portion in the figure is a portion where light leakage occurs.

As is clear from a comparison between FIG. 5 and FIG. 3, the conventional antiferroelectric liquid crystal display element shows the leakage of light per unit area when the dark display in the OFF state is enlarged and viewed. The number of places is large as shown in FIG. 5, and therefore, the entire dark display looks like whitish black, and the display brightness ratio between the ON state and the OFF state is only about 30: 1, which is low contrast. Absent.

On the other hand, in the antiferroelectric liquid crystal display element of the above-mentioned embodiment, the number of light leakage spots per unit area when the dark display in the OFF state is enlarged and seen is extremely small as shown in FIG. Therefore, since the entire dark display looks almost black, it is possible to obtain a high-contrast display with a large display brightness ratio between the ON state and the OFF state.

The display brightness ratio between the ON state and the OFF state of this antiferroelectric liquid crystal display element is approximately 95: 1 to 11
It is 0: 1, and has a contrast three times or more higher than that of a conventional antiferroelectric liquid crystal display device (contrast about 30: 1).

That is, in the antiferroelectric liquid crystal display device, the transparent electrodes 3 and 5 of the substrates 1 and 2 are most scanned on the scanning locus at a scanning distance of at least 500 nm when the surfaces thereof are scanned in an arbitrary direction. Since the height difference between the high convex portion and the lowest concave portion is an electrode having a surface roughness of 20 nm or less, it is horizontally aligned by the alignment films 6 and 7 of both substrates 1 and 2 and these alignment films 6 are formed. , 7 are arranged in a good alignment state in which the alignment state is regulated by the alignment film (alignment regulating film) 7 on the lower substrate 2 side, the liquid crystal molecules are brought into the third stable state. Arranged O
It is possible to reduce light leakage in the FF state and obtain a high-contrast display with a large display brightness ratio between the ON state and the OFF state.

In the above embodiment, the arrangement regulating film 7 on the lower substrate 2 is a rubbing-treated polyimide film, but the arrangement regulating film 7 is a monomolecular film of a polyimide precursor on the substrate 2. LB obtained by laminating the laminated film on the required layer by the LB (Langmuir-Brogit) method
It may be a film or the like.

Further, in the above embodiment, the surface of the transparent electrode (pixel electrode of the upper substrate 1) 3 below the non-alignment regulating film 6 which does not have the alignment regulating function of liquid crystal molecules is also scanned in an arbitrary direction. At this time, the height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm is 20.
Although the electrode has a surface roughness of nm or less, the transparent electrode 3 below the non-alignment regulating film 6 does not necessarily have to have the surface roughness as described above.

Further, although the antiferroelectric liquid crystal display element of the above embodiment is of the active matrix type, the present invention can be applied to, for example, a simple matrix type antiferroelectric liquid crystal display element.

The present invention can also be applied to an antiferroelectric liquid crystal display device in which the alignment films of both substrates are used as the alignment control films, and in that case, the transparent electrodes of both substrates are as described above. An electrode having various surface roughness may be used.

[0035]

In the antiferroelectric liquid crystal display device of the present invention, the horizontal alignment film on at least one substrate is an alignment control film for controlling the alignment state of liquid crystal molecules, and the transparent film under the alignment control film is transparent. The electrode is an electrode having a surface roughness in which the height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm when the surface is scanned in an arbitrary direction is 20 nm or less. Therefore, the liquid crystal molecules whose alignment state is regulated by the alignment regulation film are aligned in a good alignment state with almost no disturbance, and the liquid crystal molecules are aligned in the third stable state in the OFF state. It is possible to obtain a high-contrast display in which light leakage is reduced and the display brightness ratio between the ON state and the OFF state is large.

[Brief description of drawings]

FIG. 1 is a sectional view of an antiferroelectric liquid crystal display element showing an embodiment of the present invention.

FIG. 2 is a diagram showing a surface roughness of an opposing electrode formed on a lower substrate on which an array control film is formed, which is observed on an arbitrary straight line by an atomic force microscope.

FIG. 3 is an enlarged view showing a part of dark display in the OFF state of the antiferroelectric liquid crystal display element.

FIG. 4 is a diagram showing a surface roughness of a transparent electrode formed on both substrates of a conventional anti-ferroelectric liquid crystal display element, observed on an arbitrary straight line with an atomic force microscope.

FIG. 5 is an enlarged view showing a part of dark display in a conventional anti-ferroelectric liquid crystal display element in an OFF state.

[Explanation of symbols]

 1, 2 ... Transparent substrate 3 ... Pixel electrode 4 ... Active element 5 ... Counter electrode 6 ... Horizontal alignment film (non-alignment regulating film) 7 ... Horizontal alignment film (alignment regulating film) 9 ... Antiferroelectric liquid crystal 10, 11 ... Polarizer

Claims (1)

[Claims] 1. A pair of transparent substrates, each having a transparent electrode formed on one surface and a horizontal alignment film formed thereon, are arranged such that their electrode forming surfaces face each other, and an antiferroelectric liquid crystal is sealed between the two substrates. In the antiferroelectric liquid crystal display element, the horizontal alignment film of at least one substrate is an alignment control film that controls the alignment state of liquid crystal molecules, and the transparent electrode under the alignment control film has a surface thereof. The height difference between the highest convex portion and the lowest concave portion on the scanning locus at a scanning distance of at least 500 nm when scanning is performed in any direction is 20 nm.
An antiferroelectric liquid crystal display device, which is an electrode having the following surface roughness.